In microwave communication, parabolic antennas (dishes) must be aimed with accuracy at the source of the signals. In ground systems (such as the systems the telephone companies employ to get the telephone calls transmitted long distance), the antennas are mounted on towers and aimed while technicians listen to the signals for their strongest point. When the strongest signal is heard, the antenna is bolted in place.
When satellites began carrying microwave transmitters and repeaters, a new problem emerged; how to follow the satellites with the dishes to keep the signals strong. The first satellites used were not in geosynchronous orbit, that is, they moved about a point on the surface of the earth relative to that point. As such, the first dish antennas were simultaneously motorized in more than one direction to find and track the satellite once the satellite was located. Such systems were very expensive. Since those days, communication satellites have been placed exactly 22,300 miles above the equator and in the plane passing through the equator where they revolve around the earth exactly once every twenty-four hours. Predictively, since the earth revolves identically, the net result is that the satellite hovers over the same spot on the earth at all times.
This simplifies satellite communications since now all one had to do is to simply find the satellite and fix the dish in place pointed at the satellite. The conventional method of moving the dishes to track non-geosynchronous satellites is known as elevation-azimuth (EL-AZ) tracking. As such, there are two axes of rotation. The first, or lowest to the ground, is usually the azimuth motion, which is a direction relative to the points of the compass, i.e. north, east, south, etc. By rotation of the antenna about that axis, just as a lazy susan on the dinner table moves about an axis perpendicular to the table top, any direction on the compass may be pointed to. Then, atop this rotating platform is normally another axis which moves up and down, that is, it goes from horizon to zenith (directly overhead). If the satellite are behind your antenna (past the zenith), one could rotate the first axis 180 degrees and then adjust the elevationa portion of the mount until the satellite was found.
The problem with this kind of mount for use with geosynchronous satellites is that to move from one satellite in the belt to the next, for each shifting from one satellite to another, the antenna must be rotated in the two axes. This makes for a complex and costly control mechanism because one must know how much to move the antenna in both directions. This may be fine for major TV stations or networks that use large earth stations that cost $30,000 to $40,000, but for the small system user (home television), a simpler method was needed.
To meet this need, particularly for the home television reception, polar mounts were designed. The polar mount has two axes of rotation, but they are oriented and spaced differently. In the polar mount, the first axis is the elevation axis. The carriage which holds the other axis is rotated about the first axis (elevation axis) until the second axis points to true north (parallel to a line drawn through the earth's poles). Assuming that such polar mounts were utilized at the equator, i.e. in the plane of the geosynchronous satellite, all that is necessary is to rotate the antenna about a second axis to find all of the satellites in geosynchronous orbit (all those that are visible from our position on the earth). In this situation, the antenna would travel in an arc directly overhead from east to west or west to east.
The problem occurs when the position of the antenna is shifted, either north or south of the equator. Things get much more complicated. If one rotates the carriage as before so that the second axis is pointing to true north, the antenna (axis) would normally point at right angles to this second axis out into space but would not find any satellites because it would be following an arc in space exactly as many miles as we are from the plane of the equator. It should be remembered that this distance would be measured directly into the earth at some angles to reach the equatorial plane passing through the earth proper. At the north pole, this distance would be nearly 4,000 miles or half the diameter of the earth.
This requires, of necessity, that one make a declination correction which functions to point the antenna southward somewhat (northward in the southern hemisphere) to intersect the circle of satellites 22,300 miles in space, opposite the equator. Once this is accomplished, to find any satellites in the sector of the circle bearing the satellites at that distance about the center of the earth and within the equatorial plane, one needs to move the antenna about only one axis. As may be appreciated, a polar mount is therefore simpler and less expensive than an EL-AZ mount described initially. In fact, this is the way most home television satellite antennas are mounted and pointed. Once proper elevation control and declination control are achieved with respect to the polar mount, these controls are never changed and the antenna is moved solely in azimuth to sweep from one satellite to the other.
It is therefore a primary object of the present invention to provide an improved low cost simplified polar mount utilizing a series of tubes to provide for orientation of the antenna support relative to the polar axis, to provide the proper declination adjustments and to permit the sweep of the geosynchronous satellite sector to a selected satellite within that sector.